The invention relates to a force sensor system, especially for determining dynamically the axle load and speed, and to a circular clamp arrangement that allows simple assembly of force, pressure and acceleration sensors.
Familiar is the static weight measuring of vehicles by means of weighbridges, or with weighing devices placeable on the roadway surface and generally easily transportable. One example of these is Swiss Patent CH 597 595, which describes a weighing platform having a weighing plate resting on a base plate through hydraulic transducers. The pressure of the liquid compressed by the loading is measured. Weighing platforms operating with strain gauge elements are known also. Thus, the weighing platform described in U.S. Pat. No. 3,949,822consists of a base plate flexible under load and a weighing plate, whereby the flexure is measured by means of strain gauges. Both weighing platforms are relatively thick and provided with ramps for driving up. Obviously they are suited only for static weighing of axle loads, or with the vehicle moving over very slowly.
In view of the growing vehicular traffic, however, a modern axle load determination must be capable of being performed dynamically, i.e., without the vehicle having to reduce its speed. Already attempts have been made to determine axle load and vehicle weight by means of so-called piezo-cables let into the top surfacing of a roadway. As signal elements, piezo-cables contain piezo-foils or piezo-compound. The measuring sensitivity of these depends very much on temperature.
A measuring arrangement with a piezo-cable laid in the top surfacing is prone to instability owing to the small force-absorbing surface and the compressibility of a plastic cable, which causes varying measuring sensitivity and other measuring inadequacies, such as instability of the measuring zero. Moreover, the measuring results of the axle load and vehicle weight are too inaccurate, mostly due to the often unsatisfactory insulation resistance of piezo-cables. The object of the present invention is to provide a force measuring system which overcomes the disadvantages named and enables dynamic determination of the axle load, speed, wheelbase and gross weight, employing advantageously two force sensor systems successively spaced at a short interval. In addition the invention shows a circular clamp system that can be used in other sensor systems.
The force sensor system must be suitable for any width of roadway and installable in simple fashion, for asphalt as for concrete roads.
The problem is solved by laying in the roadway a force sensor consisting of a hollow section in which a number of piezo-elements are fitted, giving a constant force measuring sensitivity over the entire length of the hollow section and arranged to allow electrical connection of the piezo-elements in series, group wise or individually, and so that simply layable modules result, consisting of amplifier, sensor and termination units.
The same hollow section is used generally on sensors for force, pressure and acceleration equipped with piezo-elements utilizing the shear effect.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.